Slow Connection with WPA

I have a Dell Inspiron 1505 with a Dell 1390 WLAN Mini-Card. The
laptop is running Windows XP Pro (SP2) with all the latest patches.
When I connect to my wireless network with WPA encryption, the
connection is extremly slow. I get less than 1mbps. With WEP (or no
encryption), the connection is normal.

I have two other laptops (a Dell 600m and a Dell 1000) that do not
have this problems - and never did. Both of the older laptops use Dell
wireless adapters. I've tried two different routers (Netgear WGT624
and Linksys WRT54G) upgraded to the latest firmware with the same
results. I've spoken to Dell, Netgear, and Linksys. They all point the
blame to the other company.

Does anyone have any idea as to why this might be happening? Is there
a Windows something that may be causing this?

I'm desperate! Please help!

Thank you!